1. Field of the Invention
The present invention relates generally to a variable equalizer, and more specifically, to the arrangement of a waveform equalizer included in a repeater or a receiver when digital signals such as PCM signals are transmitted via paired cables or co-axial cables to a long-distance place.
2. Description of the Prior Art
Such a transmission line as a co-axial cable usually has a transmission loss due to a skin effect on the line conductors, and the transmission loss is proportional to the square root of the frequency (f) of the transmission signal. That is, the transmission line has a .sqroot.f frequency characteristic.
In order to transmit data over a long distance via a transmission line having such a characteristic, it is necessary to install variable equalizers called .sqroot.f equalizers at the terminal or intermediate points of the line to thereby compensate for the line transmission loss. In the event that .sqroot.f equalizers are used, both the gain and frequency of the transmission signals must be adjusted according to the length or distance of the transmission line in order to compensate for the .sqroot.f frequency characteristic. More specifically, the level of the input signal is detected to estimate the length of the transmission line and offer the optimum gain and frequency characteristics to the line. In a conventional PCM transmission system, .sqroot.f equalizers are inserted in the transmission line at equal intervals and each of the .sqroot.f equalizers have a narrow variable gain range. In the event that known Bode type .sqroot.f equalizers are used in a PCM transmission system such as telephone subscriber lines in which the line length ranges from 100 m to 10 Km, for example, the .sqroot.f equalizers are inserted in the lines at intervals of 1.7 Km and the equalizer has a narrow variable gain range of several dB. For this reason, many repeaters must be installed in longer line applications. In order to avoid economic undesirability of using a large number of repeaters, it might be possible to expand the variable gain range of the equalizer. In this case, since the gain range of the Bode type .sqroot.f equalizer is controlled only by one or two element values therein, the element values will decrease in accuracy as the gain ranges of the equalizer is expanded, which leads to increased errors in operation of the equalizer.
Moreover, in order to widen the gain range of the equalizer and offer a more precise equalization operation, a large sized equipment is required. This makes it impossible to practically build in or incorporate such an equalizer, for example, in a telephone set.